If you are an ultra athlete that runs or rides for 24+ hours at a time, or if you just want to record a long activity without resorting to a less accurate recording mode you can rejoice!
I've been a Garmin user for years but some of my friends use Suunto watches and one feature I was always jealous of is their ability to charge the watch while recording an activity. This allows them to use the most accurate recording mode and still log long workouts or races as a single activity.
By default the fenix 3 is set to "Mass Storage" mode. This means that when you connect your watch to a computer via the USB charging cradle it will be treated like any other USB storage device and you can browse the contents of your watch from your computer. And of course your watch will also get charged while it is connected.
If you change this setting to "Garmin" mode you will get prompted to enable Mass Storage mode every time your watch is connected to the charging cradle. All you need to do is say "No" and your watch will get charged while it keeps on recording your activity.
To change this setting go here:
Settings > System > USB Mode
So why not always leave your watch in Garmin mode? Garmin Express can't see or sync your watch when it's in Garmin mode. The advantage to selecting Mass Storage is you don't need to press two buttons to enable Mass Storage every time you connect your watch to your computer.
Of course you can still sync activities by using the Garmin Connect Mobile app on your phone and pairing your phone with your watch and you can install some updates via Wi-Fi but if you want to update apps that you have installed on your fenix 3, you need Garmin Express.